Insulating 4" wall studs - any experience of this?

[DavidB]

Hi

I am looking at options for insulation for a grade II C18 timber frame cottage. The stud posts are 4" deep and batons for the plasterboard are installed at 1" from the interior face of the frame. The beams are exposed internally, but rendered over externally. I have ripped out the insulation and plasterboard in our downstairs shower room and started making enquiries with our conservation officer about whether to remove the external cement render and re-plaster in lime.

I have looked at some insulation options online, the conservation officer mentioned wood fibre and I've also looked at hemp and wool options. I've been checking out the spec for plastering and think I need circa 1) A haired scratch coat (12-15mm) and then 2) An unhaired float coat (8-10mm) the lime plaster will sit on laths 6mm? ( not sure which I should use oak, larch, chestnut / riven,sawn...?) So that means I need to move the batons back a bit (6mm) and that will leave at best 60mm for insulation. However the external render is quite lumpy so I might not have 60mm everywhere.

Has anyone else insulated a similar building and depth of stud - what did you use and how did it work out?

 

[Zebra]

I have done something like this. 4" studs. External wall insulation over the studs with a proprietary lime render. Internally, OSB boards for racking strength and airtightness, woodwool board, lime plaster. This doesn't leave timbers exposed though. Airtightness is difficult to achieve with exposed timber, and is really important if you want to keep warm and avoid interstitial condensation.
Sorry for being really brief right now, I'm supposed to be outside doing the garden while the weather's good. I'll explain more later if I may!

 

[Zebra]

I'm not sure I understand your construction. You say the studs are exposed internally, but then you mention plasterboard. Are you going to expose the timber frame internally? Can you see that it was like that before?
If you are replacing the external render then it's a good opportunity to look at external wall insulation, depending on the window reveals and roof overhang, as it will affect all of those. I was doing everything anew, so had the opportunity to do so.
As I said, my construction was 100mm woodfibre EWI with a proprietary render. Wool between the studs. Internally I used OSB siliconed to the timber frame (silicone will come off without staining, so seemed a good way to achieve airtightness, then lime plaster on woodwool. That was a fully breathable system.
Another alternative is celotex as EWI. This system was recommended to me on this forum, I can post the link to that thread......
You could also use laths, I don't think it much matters which type.
Whereabouts in the country are you? You'd be most welcome to visit if you're in Herts any time.

 

[Zebra]

Link to thread where celotex system recommended: http://periodproperty.co.uk/forum/viewtopic.php?t=12173

 

[DavidB]

Thanks Zebra

I am in North Essex, near Saffron Walden.

Re my construction - beams are exposed internally and all painted black. Plaster board is installed throughout and nailed to batons on studs. Basically what I am saying is laths/lime is going to be thicker than plaster board and a thin skin coat, so I am going to have to move the batons back. Which means less room for insulation. I think original insulation would have been sheep's wool.

 

[DavidB]

Thank you Zebra, that link covers many of my concerns. Re external render, I too am worried that it provides more structural support than it should. I have so far found 5 posts and a corner post that either were dangling in thin air or resting on dust and rotten sole plate. So the weight must be going somewhere.
I am worried that if I take down the cement render the frame will break somewhere.
My house is listed and whilst I am only planning on repairs I have contacted the conservation officer. What I did was to make some immediate repairs to the frame to reinstate some structural support and then seek guidance from CO.
Two obvious limitations to my house for external insulation are, I) it is s semi - so need to match wall depth with neighbour or it will look different. 2) I have rie rods which were now a feature of the house - so i would want them supporting the frame not pulling in on insulation.

One of the posters on the post you linked above talked about doing the re-rendering in sections, this is the approach I suggested in my method statement to the CO.

My wife thinks I am mad and that we simply need a new shower (and quickly) whereas I am talking about stripping all the external render, replacing the internal plaster, putting in wool or wood fibre insulation, moving the ventilation fan, removing cement render from the plinth walls (internal and externally) and splicing in lumps of new oak here and there. Although I am suggesting doing it in stages I.e. internal plaster first, with new insulation that won't mind a little damp due to external cement, then fit new shower and then re-render external plinth wall in lime and re-plaster externally the timber framed walls in lime to the gable end. Then see how that goes and repeat the process on the much larger front and rear walls maybe next year and match them into our neighbours.

But I am mixing all the issues up here, sorry. My original question is about the correct depth of batons for internal lime rendering and checking what space that will leave for insulation - I suspect that if I use something like pavaflex then I will only be able to fit 50mm? I was thinking sheep wool might be more flexible to cope with internal humps and bumps of existing cement render. You might think I am doing this all backwards and that I should sort the external render first and then do the internal repairs, but I think I need to get the shower room functioning again or MrsB is not going to be happy! Not to mention the teenage son and daughter

 

[Zebra]

Like this?

Here's another thread where timber frame repairs were discussed. (Sorry to only post to my own threads, they're the ones which most readily come to mind.)
http://periodproperty.co.uk/forum/viewtopic.php?f=1&t=12565&hilit=repairs+to+timber+frame
You are mad. Quite, quite mad. So are we all, but our old houses need us, and love us for it. Normal people wouldn't go to these lengths, of course. You're in good company though. I've always found the folks on here extremely supportive when everybody around me has been saying "what on earth are you doing that for?!"
I did mine from the inside out too. It's only the upstairs which is timber frame, but I fitted the sheepswool, thus:

then the OSB sheathing board internally while the cement render was still on, then took it off:


Re moving the battens back, that sounds like quite a minor issue, and one which you've already worked out. Sheeps wool would work very well in those circumstances. If you repair the timber one elevation at a time, then the house won't collapse!
Removing the render a small section at a time is actually quite easy. I used old second hand ply boards to temporarily support and stabilise the frame while the render was off, then installed the woodwool boards one section at a time. If you don't have the room for EWI because you're a semi, then you could use 25mm woodfibre boards.
(For clarity, woodwool is beige, spongy, soft; woodfibre is grey, textured, hard. Both are breathable and suitable for lime render bases. Have a look here for the specs on the different boards: https://www.lime.org.uk/)

 

[DavidB]

Thanks Zebra, those pics and posts are great. My conservation officer seemed keen for me to use traditional methods of fixing plaster I.e. laths rather than boards, but in the same breath said wood fibre (maybe she said wood wool?) was an acceptable insulation.
I have asked the CO for some advice and am waiting for a reply.
I think (if acceptable to our CO) the wood fibre boards might be better in some ways as that means; less fixings/impact on original frame, add to insulation, and reduce cold bridging from external render to the frame.

I am now trying to find a traditional lime plaster course running this May

 

[GPHouse]

Hi DavidB

Looks like fun...

Lime plastering courses - i've just done one with Mike Wye and they have a few lime courses running may and june (plastering next is june I think). It was informative and it'll leave you capable of talking a good game... but depending on your plastering experience to date.... you're not likely to leave capable of plastering super sharp lines... but you will be able to get a traditioanl rustic feel!

The other interesting thing on thier course they covered which might be of interest is eco-cork lime plaster. It's basically a lime render and insulation in one...

G

 

[DavidB]

Thanks G, yes I have checked out that one - looks best for me due to timing.

 

[Zebra]

I did one with Ty-Mawr, and agree that the resulting effect within my plastering work is rustic, but it is very satisfying to be able to do it yourself (however long it takes! )

 

[Zebra]

(For clarity, woodwool is beige, spongy, soft; woodfibre is grey, textured, hard.

Oh blimey, dear me. I think I've got that the wrong way round.

 

[DavidB]

Hi Zebra

I was looking at this for insulation https://www.mikewye.co.uk/product/pavaflex-flexible-wood-fibre-insulation/ and savolit boards for plastering onto. But yesterday I got a local Building Conservation specialist round to quote on the external plaster. His advice was lath externally because a) the accumulative effect of all the path, especially if oak, would add strength to the frame and b) my walls are all over the place and he thought boards would give a series of planes and that a smooth curving finish would be better achieved with laths. However, he warned the oak laths, particularly riven are expensive.

 

[Zebra]

I'm sure he has a point. Also, yours is listed, isn't it? (Mine isn't.) The conservation officer may well prefer laths.
I'm just sending you a private message...

 

[GPHouse]

Sounds like good advice.

I was thinking about using savolit too, on some internal walls which neeed to be breathable. I asked a lot of questions while with the guys at MIke Wye.

Turns out the Savolit route is not easy... you end up using lots of extras, mesh, hydraulic limes etc... The advice I was given was essentially, "if you can, it's easier to stick with lath..."

Anyone have any contrary advice as I still have a hankering / think the savolit would offer me some other advantages... ?

 

[Zebra]

I used something similar to that product for internal plastering. Most of my internal ceilings and walls in fact. It was easy to use. Perhaps I cheated a little bit, because I didn't use mesh. I glued the edges of the boards to each other with gripfill, and then only did one coat of unhaired putty-based lime plaster. (shhh!!! don't tell anyone!!)
Where the structure tends to move, i.e. the timber frame upstairs, it has cracked along the joins. Where the structure is solid, i.e. brickwork behind, it hasn't.

 

[GPHouse]

Cunning like a fox... sorry, Zebra.

Sounds like a plan, tbh the mesh malarky just seems a faf. I'm still tempted but its a hard call as i have some lath still in place...

 

[andrew gayton]

Are you hoping to keep your frame visible on the interior? if not, you could simply increase the sectional thickness of the wall with an internal framework and then fill this with permeable insulation of choice, then lath and plaster on both faces. I did this and insulated with light clay straw. google light clay straw and you will get the idea.

 

[DavidB]

Thanks Andrew

Originally I had simply assumed I would keep the frame exposed internally and covered externally - cos' that is how it is now and I assumed that is how it has always been.

I am still waiting for conservation officer's response to my proposed methods statement.
In the meantime I continue to patch and repair the sole plate and stud posts in the exposed wall in the downstairs shower room. Getting up close and hands on with the frame has taught me the following:

1. There are no oak pegs anywhere as far as I can tell, instead posts are secured by rusty rosehead iron nails where they support diagonal studs and into the sole plate by varying sizes mortice and tenon
2. The face of the studs either have lots of holes in them and/or they have in some places had an 'infill' facia board fitted to them to bring out their depth to match other exposed studs - this makes me wonder if originally laths were in fact nailed to the front of many - if not all the beams and therefore when first built much less of the frame was exposed.
3. Hardly any of the existing laths on the external render have survived and they have been nailed to the studs using over sized nails and there are even the odd raw plug and screw suggesting recent repair to the render.
4. I have not yet found a stud post which has an intact tenon - so far in a shower room with 10 studs and a larger corner post - only 3 posts seem to be secure - I therefore have structural render! This render is very strong as I have been banging the posts with hammers and mallets abdprying them out with crowbars and not one crack has developed so far.

So perhaps in light of point 2 above I could argue a case for covering the studs or even some of the intermediary studs as you suggest Andrew - but it's clearly been exposed for some time. Most rooms are like this with the exception of some of the rooms upstairs which have plaster board over the rafters.

At the moment I am wrestling with 'do I strip off the black paint' or 'paint the repaired studs and new studs and sole plate with a finish to match the rest of the house'?

 

[andrew gayton]

Hello David,

Not sure where you are in the country, but not all timber frame was built to be exposed - it might well have been exposed at some point during a misinformed restoration. I would suggest that if the frame is not a thing of beauty, and is in need of significant repair, that you do give thought to concealment with a secondary frame and see the benefit this brings for thermal improvement as a positive - imagine if the frame were beautiful and intended to be expressed .... all well and good, but 100mmm is not good for obvious reasons.

if you do go down the 'concealment' route, this will of course allow you to repair the frame using partner / paired studs, rather than having to splice repair.

Despite what some might say, I am a firm believer in through permeability of historic solid walled fabric, and will use natural material/s that are tried and tested, although not always in a conventional manner.

Andrew